The present invention is directed generally to the digestion of cellulosic material such as wood chips in a batch-type process. More specifically, the present invention is directed toward an improved batch-type process for digesting cellulose which results in a kraft pulp having improved bleachability and runability qualities.
Batch processes for digesting wood chips are known. Specifically, in conventional batch processes, the digester is filled with wood chips and then charged with a digesting liquor which, in a kraft pulp producing process, includes a solution of sodium hydroxide and a sulfur compound. The digester is then sealed and the charge is heated with steam to a cooking temperature at which the charge is maintained for a period of time referred to as "the cook". At the conclusion of the cook, a blow valve in the digester is opened and the contents of the digester is discharged into a blow tank by virtue of the hot liquor in the digester flashing into steam and forcing the delignified pulp out of the digester.
The above-described process is energy inefficient because much of the heat energy acquired by the wood chips and the digesting liquor during the process is lost through the blow tank and exhaust vapors. To recover this lost heat energy, attempts have been made to pass the vapors through various forms of heat recovery systems. However, the heat recovery systems were proven to be relatively inefficient and, instead, many pulp manufacturers attempted to install continuous digestion processes. The continuous digestion processes however are not preferred because of the high cost of the equipment needed for such a process when compared to the cost of the equipment needed for batch type processes.
One solution to the lost heat energy of batch type processes was provided in the form of a displacement heating process disclosed in U.S. Pat. Nos. 4,578,149 and 4,601,787. These patents disclose modified batch processes wherein, at the end of the cook, the digester is held under pressure, a displacement liquid is used to displace the hot cooking liquor under pressure and substantially at the cooking temperature. The displacement fluid may be recycled filtrate from the washing process. The pulp is then removed from the digester with the filtrate.
For the subsequent batch, the digester is again charged with wood chips. The chips are then preheated with the warm and hot liquors that were used in the previous batches. One accumulator is used for each displacement liquor. Accordingly, three accumulators may be used to store the displaced cool, warm and hot liquors, all of which may be used in the pre-treatment of the wood chips. As a result, most of the heat energy in the spent or partially spent liquors is utilized. All of the liquor fills in the digester are done by displacement. In previously known displacement techniques, i.e. the displacing fluid is pumped into the bottom of the digester and the displaced fluid flows out of the top of the digester.
U.S. Pat. Nos. 5,059,284 and 5,080,757 disclose yet further improvements to the above-described batch processes wherein, after the cook, the black liquor (or the spent liquor) is removed and transferred to a black liquor reservoir under pressure by pumping in a lower temperature displacement liquid in both the bottom and top ends of the digester. The black or spent high temperature liquor is removed at a mid-portion of the digester. In other words, the hot black liquor is pushed out through a mid-portion of the digester due to the simultaneous pumping of cool displacement liquid into both the top and bottom of the digester. As a result, the total time required for displacement is reduced. Further, intermixing of the displacement fluid and the hot black liquor is reduced since the distance along which the hot black liquor/displacement fluid travels is reduced by approximately one half. In other words, instead of the hot black liquor/displacement fluid interface traveling the entire length of the digester, the interface travels only one half of the length of the digester, i.e. from the top of the digester to the middle of the digester or from the bottom of the digester to the middle of the digester. Further, using the improved displacement techniques disclosed in U.S. Pat. Nos. 5,059,284 and 5,080,757 permits the operator to quickly terminate the cook and avoid overcooking the wood chips while, at the same time, reducing the overall cycle time.
In the manufacture of kraft pulp using the processes disclosed in U.S. Pat. Nos. 4,578,149, 4,601,787, 5,080,757 and 5,059,284, it has been found that some pulps, including kraft pulps, may be difficult to bleach to full brightness with a moderate consumption of bleaching chemicals than other pulps. Further, the kraft pulps may be more difficult to process in other process steps, such as pumping the pulp out of the digester after completion of the cooking, screening and washing of the pulp because the pulp exhibits hydrophobic qualities.
Analyses of the kraft pulp fibers indicate that the fiber surfaces are covered with a coating of a material deposited from wood extractives. Little is known about the composition of this coating. The coating is particularly problematic if the specie of wood used as a source of the wood chips is rich in nonsoponifiable extractives. While extensive studies have been performed to obtain an understanding of the nature of the coating on the kraft pulp fibers and to find a solution of how to avoid this coating, no significant advances in addressing this problem have been provided.
Accordingly, there is a need for an improved batch process for producing kraft pulp which provides a way to produce kraft pulp fibers using the energy saving techniques provided by the processes discussed above but which also produces a kraft pulp fiber that is free or substantially free of the problematic coating. Such a process would provide an energy efficient means for producing kraft pulp fibers which have improved bleachability and runability characteristics.